In recent years, with the assistance of capable computing, it has been possible to use automated equipment to perform some of the inspection tasks that previously required a human eye. Such “machine vision” techniques have become more sophisticated, and among other things have been employed for the inspection of sheets and webs of material. Good illumination of the article being inspected is needed for the proper deployment of machine vision, especially when inspecting for surface defects on transparent or semi-transparent webs.
One technique that has been employed is the so called, “far dark field” technique, which is used to detect defects that strongly scatter light. When using the far dark field technique, one or two light sources are placed so that their light falls upon the article to be inspected (for example, a film or similar material) at an angle to the line of vision of the camera. In the absence of an upset condition, e.g. a defect, the camera will normally see no light from these sources. Only when a surface defect passes through the beams will light be reflected toward the camera so that flaws are seen as lighted areas on a normally dark image.
Another technique is the so called, “dark stripe” technique, which is used to detect defects that weakly scatter or deflect light. When using the dark stripe technique, illumination from a source is aimed through the web and directly at the camera. This illumination is diffused and attenuated with a dark stripe mask. This mask is intended to block the majority of diffuse light from the view of the camera. Only when a surface defect that reflects or refracts the light rays from their indirect paths will light be diverted towards the camera so that flaws are seen as lighted areas on a normally dark image.
Each has some advantages for highlighting the flaws on clear webs, but there are some defects that resist detection with one or the other of these methods.